The present invention relates generally to the foam molding process, and more particularly, to apparatus for directly injecting an inert gas into molten thermoplastic resin being advanced through an extruder.
The foam molding process is similar to conventional injection molding processes, except that the foamed effect is produced when inert gas is dispersed through the molten thermoplastic resin or polymer melt in the extruder. The inert gas is introduced into the polymer melt either directly or by mixing a chemical blowing agent with the thermo-plastic pellets prior to being fed into the extruder.
The polymer melt with the inert gas dispersed therein is fed into an injection chamber which holds it under a high pressure to prevent its premature expansion. When the injection chamber is filled with a predetermined quantity of the polymer melt, an injection plunger rams the melt into a mold cavity to partially fill it. Because the mold is at much lower pressure than the pressure maintained in the injection chamber, the polymer melt expands and fills the entire mold cavity forming a foamed plastic product having an integral skin formed over a structure having numerous internal cells.
Foam molding offers numerous advantages. The plastic parts produced by this process exhibit high rigidity along with a high strength to weight ratio. Also, the foam molding process enables large parts to be molded without any induced stress, and at a low cost.
The quality of the foamed molded part is direct function of the capability of the equipment to uniformly disperse the proper quantity of gas into the polymer melt. An excessive volume of gas injected into the polymer melt will cause large voids in the molded product thereby reducing its strength and creating visually unacceptable imperfections in the external skin of the part. On the other hand, an insufficient volume of gas will cause the molded part to be denser than is required thus nullifying the benefits of a foamed article, that is, its light weight coupled with high strength.
One common method of introducing gas to a polymer melt utilizes a gas compressor, a gas storage tank connected to the discharge of the gas compressor and a pressure regulating valve disposed in a conduit connecting the gas storage tank with the extruder. The pressure differential method utilizing this apparatus provides for the compressor to deliver gas to the storage tank at extremely high pressures, 3000 psi and higher. The pressure regulating valve is set in the 1500-3000 psi range depending on the requirements of the plastic material and the extruder capacity. The gas flows into the extruder as a result of a pressure differential between the regulator valve and the polymer pressure generated in the extruder chamber. A small differential between these two pressures produces a low volumetric flow rate of the gas to the polymer melt and a large differential between these pressures produces a high volumetric flow rate. The main disadvantage with this method is that the gas flow into the extruder does not remain constant because of the pressure fluctuations in the extruder chamber. These pressure fluctuations are of either a short time span caused by surges of the polymer melt in the extruder or of a long time span caused by long standing changes in the extruder back pressure, temperature and polymer viscosity. Accordingly, these pressure fluctuations vary the pressure differential between the regulator valve and the pressure in the extruder chamber causing either too much or too little gas to be dispersed throughout the molten thermoplastic resin. This uneven gas distribution in the polymer melt causes the molded article to exhibit poor foamed characteristics.
Recently, efforts have been made to provide for the uniform dispersion of the foaming gas throughout the polymer melt. U.S. Pat. No. 4,043,715 discloses a pumping system consisting of a single stage diaphragm compressor driven by a variable speed drive at an adjustable rate. The valve which delivers the gas to the extruder cooperates with the compressor to supply a train of discrete gas pulses. In this manner, discrete gas bubbles are supplied to the polymer melt in an attempt to uniformly disperse the gas throughout the polymer. Such a system fails to respond quickly to the changing volumetric gas requirements in the extruder. A variable speed compressor thus becomes an operational liability.
U.S. Pat. No. 3,981,649 discloses a system in which a constant volumetric flow rate of gas is delivered to the extruder. The gas flow apparatus disclosed in this patent uses a self-regulating valve that automatically responds to pressure changes in the extruder chamber to maintain a constant pressure upstream of itself which results in a constant flow rate of gas to the extruder head. In this patent, the self-regulating valve operates to maintain a constant pressure drop across the valve in order to maintain a constant flow rate through the valve into the extruder head. The flexibility of the system, however, is hampered in that it cannot be readily regulated to accomodate a wide variety of operating conditions. Furthermore, the gas supply must be maintained at undesirably high pressures, creating a dangerous environment for workers.
The objects of the present invention are to provide: apparatus for controlling the volumetric rate of gas into a molten thermoplastic resin in an extruder chamber for producing a foamed thermoplastic resin article having a uniform cell size and distribution; an apparatus which will automatically compensate for pressure variations of the molten resin in the extruder in maintaining a constant volumetric flow rate of gas to the molten resin; an apparatus which will accept a supply gas at an extremely low pressure value and increase this gas to a usable high pressure value in order to provide a safe environment; apparatus that will continue operating when the extruder is shut down; an apparatus which efficiently measures the volumetric gas flow into the extruder chamber and which enables the adjustable regulation of the amount of gas flow into the extruder with a fixed output compressor; and an apparatus which is rugged in construction and capable of operating continuously over long periods of time with a high degree of reliability.